Silicon etch process with tunable selectivity to SiO2 and other materials

ABSTRACT

A tunable plasma etch process includes generating a plasma in a controlled flow of a source gas including NH 3  and NF 3  to form a stream of plasma products, controlling a flow of un-activated NH 3  that is added to the stream of plasma products to form an etch gas stream; and controlling pressure of the etch gas stream by adjusting at least one of the controlled flow of the source gas and the flow of un-activated NH 3  until the pressure is within a tolerance of a desired pressure. An etch rate of at least one of polysilicon and silicon dioxide by the etch gas stream is adjustable by varying a ratio of the controlled flow of the source gas to the flow of un-activated NH 3 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application of, and claims thebenefit of priority to U.S. Provisional Patent Application No.62/054,860, filed 24 Sep. 2014 and entitled “Silicon Etch Process WithTunable Selectivity to SiO₂ and Other Materials,” which is incorporatedherein by reference in its entirety for all purposes.

TECHNICAL FIELD

The present disclosure applies broadly to the field of plasma processingequipment. More specifically, systems and methods for managingselectivity of plasma etches are disclosed.

BACKGROUND

Semiconductor processing often utilizes plasma processing to etch, cleanor deposit material on semiconductor wafers. Plasma etching oftentargets a particular material, but may also affect other materials thatare present on the same wafer, or other workpiece. Selectivity of aplasma etch is often defined as a ratio of an etch rate of a targetmaterial to the etch rate of the same etch to another material. It isoften advantageous for selectivity to be very high, such that the targetmaterial can be etched with little effect on other materials.

SUMMARY

In an embodiment, a tunable plasma etch process includes generating aplasma in a controlled flow of a source gas including NH₃ and NF₃ toform a stream of plasma products, controlling a flow of un-activated NH₃that is added to the stream of plasma products to form an etch gasstream; and controlling pressure of the etch gas stream by adjusting atleast one of the controlled flow of the source gas and the flow ofun-activated NH₃ until the pressure is within a tolerance of a desiredpressure. An etch rate of at least one of polysilicon and silicondioxide by the etch gas stream is adjustable by varying a ratio of thecontrolled flow of the source gas to the flow of un-activated NH₃.

In an embodiment, a plasma etch processing system that etches siliconand silicon dioxide with tunable selectivity includes a first NH₃ flowcontroller, an NF₃ flow controller, and a remote plasma sourceconfigured to apply RF energy to a plasma source gas stream controlledby the first NH₃ flow controller and the NF₃ flow controller, togenerate a plasma product stream from the plasma source gas stream. Theprocessing system further includes a second NH₃ flow controllerconfigured to control an un-activated NH₃ gas stream, apparatus foradding the un-activated NH₃ gas stream to the plasma product stream toform an etch gas stream, and a process chamber configured to expose aworkpiece to the etch gas stream. Etch rates of at least one ofpolysilicon and silicon dioxide by the etch gas stream on the workpieceare adjustable at least by varying a ratio of the plasma source gasstream to the un-activated NH₃ gas stream.

In an embodiment, a plasma etch processing system includes a processchamber configured to expose a workpiece to an etch gas stream, and aplasma source that generates a plasma product stream from a source gasstream that includes NH₃ and NF₃. The system also includes means forcontrolling a first NH₃ flow and an NF₃ flow of the source gas stream,and means for controlling a second NH₃ flow of an un-activated NH₃ gasstream. The system also includes a diffuser plate disposed between theplasma source and the process chamber that allows the plasma productstream to flow through the diffuser plate toward the process chamber,and adds the un-activated NH₃ gas stream only on a process chamber sideof the diffuser plate, to form an etch gas stream. The system alsoincludes a controller for controlling the plasma source, the means forcontrolling the first NH₃ flow and the NF₃ flow of the source gasstream, and the means for controlling the second NH₃ flow of theun-activated NH₃ gas stream, such that the controller adjusts etch ratesof at least one of polysilicon and silicon dioxide by the etch gasstream on the workpiece by varying a ratio of the un-activated NH₃ gasstream to the source gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates major elements of a plasma processingsystem, according to an embodiment.

FIG. 2 schematically illustrates major elements of a plasma processingsystem, in a cross-sectional view, according to an embodiment.

FIG. 3 is a flowchart of a tunable plasma etch process, according to anembodiment.

FIG. 4 shows a graph that illustrates test etch rate results of a Sietch having tunable selectivity to SiO₂, according to an embodiment.

FIG. 5 illustrates an SiO₂ region on a wafer section, forming trenchesthat are lined with TiN, according to an embodiment.

FIG. 6 illustrates the SiO₂ region of FIG. 5 as including a significantfraction of Si, according to an embodiment.

FIG. 7 illustrates the result of etching the wafer section of FIG. 5with a Si etch having tunable selectivity to SiO₂ to form a modifiedwafer section, according to an embodiment.

FIGS. 8A, 8B and 8C schematically illustrate a process sequence in whichtrace amounts of SiO₂ are not successfully removed by prior artprocessing.

FIGS. 9A, 9B, 9C and 9D schematically show how the structure shown inFIGS. 8A, 8B and 8C can be more successfully processed with a Si etchwith tunable selectivity to oxide, according to embodiments herein.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates major elements of a plasma processingsystem 100, according to an embodiment. System 100 is depicted as asingle wafer, semiconductor wafer plasma processing system, but it willbe apparent to one skilled in the art that the techniques and principlesherein are applicable to plasma generation systems of any type (e.g.,systems that do not necessarily process wafers or semiconductors). Itshould also be understood that FIG. 1 is a simplified diagramillustrating only selected, major elements of system 100; an actualprocessing system will accordingly look different and likely containadditional elements as compared with system 100.

Processing system 100 includes a housing 110 for at least a waferinterface 115, a user interface 120, a plasma processing unit 130, acontroller 140, one or more flow controller(s) 156 and one or more powersupplies 150. Processing system 100 is supported by various utilitiesthat may include gas(es) 155, electrical power 170, vacuum 160 andoptionally others; within system 100, controller 140 may control use ofany or all of such utilities. Internal plumbing and electricalconnections within processing system 100 are not shown, for clarity ofillustration.

Processing system 100 is illustrated as a so-called indirect, or remote,plasma processing system that generates a plasma in a first location anddirects the plasma and/or plasma products (e.g., ions, molecularfragments, free radicals, energized species and the like) to a secondlocation where processing occurs. Thus, in FIG. 1, plasma processingunit 130 includes a remote plasma source 132 that supplies plasma and/orplasma products for a process chamber 134. Process chamber 134 includesone or more wafer pedestals 135, to which wafer interface 115 deliversand retrieves workpieces 50 (e.g., a semiconductor wafer, but could be adifferent type of workpiece) for processing. Wafer pedestal 135 isheated and/or cooled by a wafer heating/cooling apparatus 117 whichcould include, in embodiments, resistive heaters, other types ofheaters, or cooling fluids. In operation, gas(es) 155 are introducedinto plasma source 132 and a radio frequency generator (RF Gen) 165supplies power to ignite a plasma within plasma source 132. Plasmaand/or plasma products pass from plasma source 132 through at least adiffuser plate 137 to process chamber 134, where workpiece 50 isprocessed. An actual plasma system may provide many other optionalfeatures or subsystems through which plasma and/or plasma products flowand/or mix between plasma source 132 and process chamber 134, and mayinclude sensors 118 that measure parameters such as pressures,temperatures, optical emissions and the like at wafer pedestal 135,within process chamber 134 as shown, and possibly at other locationswithin processing system 100.

The elements illustrated as part of system 100 are listed by way ofexample and are not exhaustive. Many other possible elements, such as:pressure and/or flow controllers; gas or plasma manifolds ordistribution apparatus; ion suppression plates; electrodes, magneticcores and/or other electromagnetic apparatus; mechanical, pressure,temperature, chemical, optical and/or electronic sensors; wafer or otherworkpiece handling mechanisms; viewing and/or other access ports; andthe like may also be included, but are not shown for clarity ofillustration. In particular, flow controllers may be, or include, massflow controllers, valves, needle valves, and pressure regulators.Various control schemes affecting conditions in process chamber 105 arepossible. For example, a pressure may be maintained by monitoring thepressure in process chamber 134 and adjusting all gas flows upwards ordownwards until the measured pressure is within some tolerance of adesired pressure. In embodiments herein, the pressure may becontrollable within a range of about 0.5 Torr to 10 Torr; in certainembodiments, controlling pressure around 2 Torr or 6 Torr may beadvantageous. Temperatures can be controlled by adding heaters andtemperature sensors to process chamber 134 and/or wafer pedestal 135.Optical sensors may detect emission peaks of plasmas as-generated and/oras they interact with workpieces.

Internal connections and cooperation of the elements illustrated withinsystem 100 are also not shown for clarity of illustration. In additionto RF generator 165 and gases 155, other representative utilities suchas vacuum 160 and/or general purpose electrical power 170 may connectwith system 100. Like the elements illustrated in system 100, theutilities illustrated as connected with system 100 are intended asillustrative rather than exhaustive; other types of utilities such asheating or cooling fluids, pressurized air, network capabilities, wastedisposal systems and the like may also be connected with system 100, butare not shown for clarity of illustration. Similarly, while the abovedescription mentions that plasma is ignited within remote plasma source132, the principles discussed below are equally applicable to so-called“direct” plasma systems that create a plasma in a the actual location ofworkpiece processing.

Although an indirect plasma processing system is illustrated in FIG. 1and elsewhere in this disclosure, it should be clear to one skilled inthe art that the techniques, apparatus and methods disclosed herein mayalso be applicable to direct plasma processing systems—e.g., where aplasma is ignited at the location of the workpiece(s). Similarly, inembodiments, the components of processing system 100 may be reorganized,redistributed and/or duplicated, for example: (1) to provide a singleprocessing system with multiple process chambers; (2) to providemultiple remote plasma sources for a single process chamber; (3) toprovide multiple workpiece fixtures (e.g., wafer pedestals 135) within asingle process chamber; (4) to utilize a single remote plasma source tosupply plasma products to multiple process chambers; and/or (5) toprovide plasma and gas sources in serial/parallel combinations such thatvarious source gases may be activated (e.g., exist at least temporarilyas part of a plasma) zero, one, two or more times, and mixed with othersource gases before or after they enter a process chamber, and the like.Gases that have not been part of a plasma are sometimes referred to as“un-activated” gases herein.

FIG. 2 schematically illustrates major elements of a plasma processingsystem 200, in a cross-sectional view, according to an embodiment.Plasma processing system 200 is an example of plasma processing unit130, FIG. 1. Plasma processing system 200 includes a process chamber 205and a plasma source 210. As illustrated in FIG. 2, plasma source 210introduces gases 155(1) directly, and/or gases 155(2) that become plasmaproducts in a further remote plasma source 202, as plasma input gas 212,through an RF electrode 215. RF electrode 215 includes (e.g., iselectrically tied to) a first gas diffuser 220 and a face plate 225 thatserve to redirect flow of the source gases so that gas flow is uniformacross plasma source 210, as indicated by small arrows 226. Afterflowing through face plate 225, an insulator 230 electrically insulatesRF electrode 215 from a diffuser 235 that is held at electrical ground(e.g., diffuser 235 serves as a second electrode counterfacing faceplate 225 of RF electrode 215). Surfaces of RF electrode 215, diffuser235 and insulator 230 define a plasma generation cavity where acapacitively coupled plasma 245 is created when the source gases arepresent and RF energy is provided through RF electrode 215. RF electrode215 and diffuser 235 may be formed of any conductor, and in embodimentsare formed of aluminum (or an aluminum alloy, such as the known “6061”alloy type). Surfaces of face plate 225 and diffuser 235 that face theplasma cavity or are otherwise exposed to reactive gases may be coatedwith yttria (Y₂O₃) or alumina (Al₂O₃) for resistance to the reactivegases and plasma products generated in the plasma cavity. Insulator 230may be any insulator, and in embodiments is formed of ceramic.

Plasma products generated in plasma 245 pass through diffuser 235 thatagain helps to promote the uniform distribution of plasma products, andmay assist in electron temperature control. Upon passing throughdiffuser 235, the plasma products pass through a further diffuser 260that promotes uniformity as indicated by small arrows 227, and enterprocess chamber 205 where they interact with workpiece 50, such as asemiconductor wafer, atop wafer pedestal 135. Diffuser 260 includesfurther gas channels 250 that may be used to add one or more additional,un-activated gases 155(3) to the plasma products as they enter processchamber 205, as indicated by very small arrows 229. Diffuser 260provides un-activated gases 155(3) only on a downstream, or processchamber side, as shown.

Embodiments herein may be rearranged and may form a variety of shapes.For example, RF electrode 215 and diffuser 235 are substantiallyradially symmetric in the embodiment illustrated in FIG. 2, andinsulator 230 is a ring disposed against peripheral areas of face plate225 and diffuser 235, for an application that processes a circularsemiconductor wafer as workpiece 50. However, such features may be ofany shape that is consistent with use as a plasma source. Moreover, theexact number and placement of features for introducing and distributinggases and/or plasma products, such as diffusers, face plates and thelike, may also vary. For example, it would be possible to substitute anarrangement for generating an inductively coupled plasma, for thecapacitively coupled plasma arrangement illustrated. Also, in a similarmanner to diffuser 260 including gas channels 250 to add un-activatedgas 155(3) to plasma products from plasma 245 as they enter processchamber 205, other components of plasma processing system 200 may beconfigured to add or mix gases 155 with other gases and/or plasmaproducts as they make their way through the system to process chamber205.

In semiconductor processing, plasma etch processes for etchingpolycrystalline silicon (hereinafter sometimes referred to as“polysilicon” or “poly”) are often advantageously selective to siliconover silicon dioxide (hereinafter sometimes referred to as “oxide”). Oneprocess scenario in which a plasma etch is highly selective to poly overoxide is as follows. Polysilicon is often utilized as a “gate” electrodein so-called “MOS” transistors (MOS standing forMetal-Oxide-Semiconductor, although poly, instead of metal, is usuallyused for the gate electrode material). To form planar MOS transistors, a“source” and “drain” are formed in a silicon wafer, with the gatedefining separation of, and influencing electrical conduction between,the source and drain. A poly layer may be deposited over a dielectriclayer, typically SiO₂. The SiO₂ layer is typically very thin, tomaximize field strength in this layer in the final transistor when avoltage is applied. The poly layer is typically thicker than the SiO₂layer, and poly depositions are often conformal such that minor creviceson the wafer surface fill with poly. When the poly layer is patterned,the poly in these crevices must be removed to prevent adjacenttransistor gates from shorting to one another. The SiO₂ layer may needto protect other parts of the wafer during the fabrication process,especially as portions of the poly layer are etched away to form thetransistor gates. Therefore, a poly etch with low selectivity to oxide(e.g., a poly etch that etches oxide, as well) might etch through theSiO₂ layer and damage the underlying silicon wafer areas that areintended to form the source and drain of the transistor.

There are, however, device geometries and processes that benefit from aless selective poly etch. For example, recent technologies have movedaway from the original, planar MOS process in which gates always overliethin dielectric layers on a generally flat silicon wafer surface, and/orform capacitors with conductors across a thin dielectric layer. Certainprocesses etch trenches into silicon surfaces, then grow or depositoxides on the trench surfaces to form trench devices, including trenchcapacitors having much larger capacitance per unit area than would bepossible on the flat wafer surface. These and/or other processesgenerate silicon fins by, for example, generating vertical steps on awafer, depositing poly conformally, and anisotropically etching back thepoly in a vertical direction, clearing the poly from horizontal surfacesbut leaving the fins behind as residual features against the verticalsteps. Many other arrangements are in use or under study to reduce waferarea needed for fabrication of complex devices, to improve deviceperformance, density and/or yield of working circuits per wafer. In suchcases, a plasma etch having a selectivity that could be adjustable, or“tunable” from highly selective to poly over oxide, to less selective,or even to being selective to oxide over poly, is advantageous.

Plasma etch embodiments herein have tunable selectivity, for exampleselectivity to Si over oxide. These embodiments typically utilize NF₃and NH₃ gases as plasma source gases, and/or as gases that are mixedwith plasma products before reaching the workpiece. Certain other gasessuch as He or Ar may additionally be used as carrier gases but generallydo not take part in reactions; also, small percentages (less than about10% of total gas flow) of oxygen may be added to help with plasmaignition and to boost reaction rates in the plasma. Generally speaking,in the plasma environment NF₃ and NH₃ dissociate, and/or some of thegenerated products thereof can combine, to form plasma products such asfree H and F radicals, HF, N₂, NH₄F and/or NH₄F.HF. Of these plasmaproducts, plasma activation favors formation of free H and F radicals,HF and N₂, while addition of un-activated NH₃ favors formation of NH₄Fand/or NH₄F.HF. When Si (e.g., polysilicon) is present, F and Si canreact to form SiF₄, while H and Si can react to form SiH₄. When SiO₂ ispresent, NH₄F or NH₄F.HF can react with SiO₂ to form (NH₄)₂SiF₆ and H₂O;importantly, (NH₄)₂SiF₆ generally forms and remains as a solid, but heatcan cause it to dissociate to form SiF₄, NH₃ and HF. Also when SiO₂ ispresent, NH₄F and NH₄F.HF can react with SiO₂ to form SiH₄F, H₂O andNH₃. Certain of these source gases may participate in reactions evenwhen provided in un-activated form, for example, when provided asun-activated gas flow 155(3), in addition to plasma products from plasma245, FIG. 2.

Etches described herein are also tunable with respect to selectivity ofpoly over silicon nitride. (The classic formulation of silicon nitrideis Si₃N₄, but stoichiometry of silicon nitride can vary depending on theconditions in which it is generated. In this disclosure, Si₃N₄ and otherstoichiometric variations of silicon nitride are referred to hereinsimply as SiN.) When SiN is present, NF₃ and SiN can react to form NFand SiF₄, while NF₃, NH₃ and SiN can react to form SiH₄.HF and NH₄F.HF.

It has been discovered that in embodiments, certain ones of the inputgases (e.g., NF₃ and NH₃) have different effects on the etching, as doesthe proportion of input gases that are activated by forming plasmatherefrom, to un-activated gases (e.g., the proportion of input gas 212to gas 155(3), see FIG. 2), so as to provide etches with tunableselectivity. Temperature and pressure have also been discovered to haveeffects. These effects can be utilized to tune an etch thatfundamentally uses the same types of input gases to have differentselectivities, as discussed below. As noted above, it is possible toadjust ratios of plasma products to un-activated gas (e.g., by adjustingratios of input gas 212, which forms plasma 245, to gas 155(3), see FIG.2) to further vary these effects. Furthermore, in embodiments, theetches disclosed can be performed in such a way as to maintain very highselectivity to other materials that may be present on a wafer and shouldbe left undisturbed, such as TiN or W. Still furthermore, the etchesdisclosed can, in embodiments, have differing etch rates such that anetch recipe can be tuned for high etch rate during one part of an etchsequence, and a different etch rate during another part of the etchsequence.

In an embodiment, a first variation of a plasma etch process (or“recipe”) has an initial ratio of NH₃ to NF₃ as a plasma source gas(e.g., input gas 212) that forms plasma products in plasma 245, and addsfurther NH₃ as an un-activated gas (e.g., gas 155(3)) resulting in aratio of input gas 212 to gas 155(3) of about 10:1. The first recipe isperformed at a pressure of 2 Torr at the wafer (e.g., workpiece 50 inprocess chamber 205). The pressure of 2 Torr may be maintained, forexample, by monitoring the pressure in process chamber 205 and adjustingall gas flows upwards or downwards until the measured pressure is withinsome tolerance of 2 Torr. The first recipe has a poly over oxideselectivity of about 20:1. In the first recipe, the dominant plasmareactions include NF₃ and NH₃ providing substantial amounts of free Fand H radicals, which react aggressively with Si but less aggressivelywith SiO₂.

A second variation of the etch recipe flows the same ratio of NH₃ to NF₃as input gas 212, but at a lower volume, and/or adds more NH₃ asun-activated gas 155(3), as compared to the first recipe. The gas flowsof the second etch recipe result in a ratio of input gas 212 to gas155(3) of about 4:1 or 5:1. The second etch recipe is also performed ata pressure of 2 Torr. The resulting poly over oxide etch rateselectivity is about 2:1. The reduced etch rate selectivity is due tothe adjusted gas flows favoring production of NH₄F and NH₄F.HF, whichsubstantially etch SiO₂, over production of F and H radicals, whichpreferentially etch Si.

A third variation of the etch recipe flows the same ratio of NH₃ to NF₃as input gas 212, and NH₃ as un-activated gas 155(3), as compared to thesecond recipe, but is performed at a pressure of 6 Torr. The increasedpressure reduces the mean free path of available F and H radicals, whichagain favors the etching of SiO₂ over the etching of Si. The third etchrecipe actually etches oxide faster than poly, with a resulting etchrate selectivity of oxide over poly of about 3:1.

The etch recipes noted above may proceed faster at relatively high wafertemperatures, such as greater than 100 C. The etch recipes noted abovecan also be highly selective to materials such as TiN and W that arecommonly utilized in semiconductor processing at the so-called“frontend” of the process (e.g., layers where transistors and gates areformed and interconnected). That is, the etch rates of TiN and W can benegligibly low. Achieving high selectivity to TiN requires thattemperature of a wafer being etched be maintained at less than about 125C, at which temperature fluorine radicals begin to attack TiN.

Because the etches described above continuously vary the ratio of NH₃and NF₃ plasma products (e.g., free F and H radicals) to un-activatedNH₃ that is added, and the pressure at the workpiece (e.g., in processchamber 205), it is believed that the selectivity of poly to oxide etchrates can be continuously adjusted at least between the extremes notedabove; that is, at least from 20:1 poly over oxide to 3:1 oxide overpoly. This generates new possibilities for etch recipes, for example,the ability to tailor a plasma etch to provide a selectivity thataccommodates process requirements for certain device geometries, or tochange the etch selectivity to accommodate changes in device geometry.It may also simplify the deployment of plasma equipment for multiple,similar etch steps in semiconductor process flows, since the inputchemistry utilizes the same gases in different ratios to producedifferent results. Because the types of input gases do not change,process aspects such as conditioning of chamber surfaces is expected notto change significantly, such that a single piece of processingequipment could be shifted rapidly from one process to another withlittle or no re-conditioning of the surfaces.

FIG. 3 is a flowchart of a tunable plasma etch process 300. Plasma etchprocess 300 may be implemented, for example, by plasma processing system100, FIG. 1, and/or other plasma processing systems that utilize plasmaprocessing system 200, FIG. 2. In step 302, a plasma is generated in acontrolled flow of a source gas that includes NH₃ and NF₃. An example ofstep 302 is flowing a mixture of NH₃ and NF₃ as input gas 212, FIG. 2,and generating plasma 245 therefrom. Plasma 245 will include plasmaproducts such as H and F free radicals, HF and N2, as shown in reaction(1) above. In step 306, a controlled flow of un-activated NH₃ is addedto the stream of plasma products, to form an etch gas stream. An exampleof step 306 is flowing NH₃ as un-activated gas 155(3), FIG. 2. Step 310controls pressure of the etch gas stream by adjusting the source gas andun-activated NH₃ flows until the etch gas pressure is within a toleranceof a desired pressure. An example of step 310 is controlling pressure ofthe etch gas stream in process chamber 205, FIG. 2, and adjusting flowsof input gas 212 and un-activated gas 155(3) until the pressure inprocess chamber 205 is within a tolerance of the desired pressure. Step312 adjusts a ratio of source gas to un-activated NH₃ to vary etch ratesof polysilicon and/or silicon dioxide to achieve the desiredselectivity. Step 312 is done in concert with step 310; that is, anadjustment to one of the source gas and un-activated NH₃ will typicallybe performed with an adjustment to the other, so that the etch gaspressure and flow ratio are maintained simultaneously.

FIG. 4 shows a graph 330 that illustrates test etch rate results of a Sietch having tunable selectivity to SiO₂. In the etch illustrated, NF₃and NH₃ may be supplied either as source gases 155(2) in the apparatusillustrated in FIG. 2, and activated in remote plasma source 202, or maybe supplied as source gases 155(1) and activated in plasma 245. In thetest data illustrated, TiN etch rate was measured but remained at zeroacross all tested process conditions. When the un-activated NH₃ flowrate was zero, Si had a low etch rate while SiO₂ (labeled as “Oxide” ingraph 330) had an etch rate of almost zero. Therefore with anun-activated NH₃ flow rate of zero, the etch conditions were extremelyselective to Si over oxide. As the un-activated NH₃ flow rate increased,both Si and oxide etch rates increased, therefore the corresponding etchconditions were less selective to Si over oxide. At the highestun-activated NH₃ flow rate tested, the oxide etch rate increased enoughthat the Si over oxide selectivity was only about 2:1.

FIGS. 5, 6 and 7 illustrate a process application that advantageouslyutilizes a Si etch with tunable selectivity to oxide, as describedherein.

FIG. 5 illustrates an SiO₂ region 350 on a wafer section 340, formingtrenches 380, 385 that are lined with TiN 360. Atop TiN layer 360 is apoly layer 370. A region labeled as A in FIG. 5 is illustrated ingreater detail in FIG. 6.

FIG. 6 illustrates that poly layer 370 sits atop a region of SiO₂ 375that includes a significant fraction of Si. It is important in thisprocess that SiO₂ 375 not be etched significantly, so when SiO₂ 375 isexposed, an etch that is highly selective to poly over oxide should beemployed, to minimize attack on SiO₂ 375. Thus, the following discussionbegins with generating a plasma in a controlled flow of source gas thatincludes NH₃ and NF₃, to form a stream of plasma products, and adding acontrolled flow of un-activated NH₃ to form an etch gas stream thatinteracts with poly layer 370 and SiO₂ 375. Referring to FIG. 4, thissuggests processing with an un-activated NH₃ flow rate of zero, ornearly zero, to maximize selectivity of the etch to poly over oxide.However, low flows of NH₃ are also associated with somewhat lower Sietch rate, which will make the etch process take longer if the low orzero flow rate of un-activated NH₃ is used continually. A compromisethat provides high etch throughput and high selectivity when SiO₂ 375 isexposed, is provided by utilizing the etch with tunable selectivity asdescribed herein. Since the poly etch rate is at least approximatelyknown from graph 330, and the thickness of poly layer 370 is known, theetch can start out with a relatively high un-activated NH₃ flow rate.For example, a flow rate of 150 to 250, in the arbitrary unitsillustrated in FIG. 4, can be used. This provides a high poly etch rate,and this etch rate is maintained for a time calculated as correspondingto etching partially, but not completely, through the region of poly 370labeled as 390 in FIG. 6. Then, either in a separate etch step orwithout interrupting the etch, the un-activated NH₃ flow rate is broughtdown to zero, or near zero, while etching the region of poly 370 labeledas 395. This causes the poly etch rate to drop, but the correspondingoxide etch rate also drops to near zero, as illustrated in FIG. 4, sothe etch can completely clear poly 370 without significant attack on Sirich SiO₂ 375. All of the process conditions are also highly selectiveto TiN such that a reasonable overetch can be used to clear poly intopologically difficult locations such as sidewalls of trenches 380 and385 while leaving TiN 360 intact. FIG. 7 illustrates the result ofetching wafer section 340 to form wafer section 340′.

FIGS. 8A, 8B and 8C schematically illustrate a process sequence in whichtrace amounts of SiO₂ are not successfully removed by prior artprocessing. FIG. 8A shows a wafer section 400 that includes a portion ofa silicon substrate 410 and overlying structures. A spacer structure 450is part of a fin-FET process. Certain recesses within spacer structure450 contain inter-layer dielectric oxides 430 that are generally to beretained in the etch sequence to be described. Other recesses containpolysilicon 420 that is to be removed; however the polysilicon ispartially oxidized along grain boundaries thereof, shown as oxide traces425, near the surface. Depth of oxide traces 425 is known withreasonable accuracy. Below the poly, adjacent to silicon substrate 410in the same recesses of spacer structure 450 are thin layers of highdielectric constant oxide (“high-K oxide” hereinafter) 440 that are notto be etched at all.

FIG. 8B schematically illustrates partial etching of wafer section 400with a highly selective poly etch to form a partially processed wafersection 401. The poly etch used is highly selective so as not to etchhigh-K oxide 440, but because of this selectivity, oxide traces 425 arealso not etched. Wafer section 401 shows how wafer section 401 will lookas poly 420 is about one third etched away.

FIG. 8C schematically illustrates further etching of wafer section 400with a highly selective poly etch to form a partially processed wafersection 402. Some oxide traces 425 remain in their original locations,while others, having lost all mechanical connection with spacerstructure 450, migrate about the recesses to other locations, as shown.At least some of oxide traces 425 cause electrical defects in thefinished semiconductor product, as they interfere with the structuresthat should be present at their locations, and/or causephotolithographic defects in further processing.

FIGS. 9A, 9B, 9C and 9D schematically show how the structure shown aswafer section 400 can be more successfully processed with a Si etch withtunable selectivity to oxide, according to embodiments herein. Thus, thefollowing discussion begins with generating a plasma in a controlledflow of source gas that includes NH₃ and NF₃, to form a stream of plasmaproducts, and adding a controlled flow of un-activated NH₃ to form anetch gas stream that interacts with the features of wafer section 400.FIG. 9A shows wafer section 400 (identical to that shown in FIG. 8A).FIG. 9B illustrates partial etching of wafer section 400 with the etchgas stream, to form a partially processed wafer section 403. The etchgas stream used to form wafer section 403 includes enough NH₃ to providea nonzero oxide etch rate, that is, to make the etch only partiallyselective to poly over oxide. As shown in FIG. 9B, oxide traces 425 areetched along with poly 420 in this part of the etch process. Once theetch penetrates past oxide traces 425 (for example, after a time atwhich a measured or calculated etch rate ensures etching to or beyondthe known maximum depth of oxide traces 425) NH₃ flow can be reduced tozero or near zero, to increase selectivity of poly over oxide. FIG. 9Cillustrates partial etching of wafer section 400 with the etch, to forma partially processed wafer section 403. The etch proceeds in thismanner until poly 420 is completely removed, as shown in wafer section405, FIG. 9D. The high selectivity of the Si etch having tunableselectivity to oxide ensures that high-K oxide 440 is not damaged aspoly 420 is removed.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Additionally, a number of well-known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent invention. Accordingly, the above description should not betaken as limiting the scope of the invention.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassed.The upper and lower limits of these smaller ranges may independently beincluded or excluded in the range, and each range where either, neitheror both limits are included in the smaller ranges is also encompassedwithin the invention, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsoincluded.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise. Thus, for example, reference to “a process” includes aplurality of such processes and reference to “the electrode” includesreference to one or more electrodes and equivalents thereof known tothose skilled in the art, and so forth. Also, the words “comprise,”“comprising,” “include,” “including,” and “includes” when used in thisspecification and in the following claims are intended to specify thepresence of stated features, integers, components, or steps, but they donot preclude the presence or addition of one or more other features,integers, components, steps, acts, or groups.

We claim:
 1. A tunable plasma etch process, comprising: generating aplasma in a controlled flow of a source gas including NH₃ and NF₃ toform a stream of plasma products; controlling a flow of un-activated NH₃that is added to the stream of plasma products to form an etch gasstream; and controlling pressure of the etch gas stream by adjusting atleast one of the controlled flow of the source gas and the flow ofun-activated NH₃ until the pressure is within a tolerance of a desiredpressure; wherein an etch rate of at least one of polysilicon andsilicon dioxide by the etch gas stream is adjustable by varying a ratioof the controlled flow of the source gas to the flow of un-activatedNH₃.
 2. The tunable plasma etch process of claim 1, wherein the etchrates of polysilicon and silicon dioxide by the etch gas stream areadjustable at least across a range of: 20:1 selectivity of thepolysilicon etch rate over the silicon dioxide etch rate, to 2:1selectivity of the polysilicon etch rate over the silicon dioxide etchrate.
 3. The tunable plasma etch process of claim 1, wherein adjustingthe ratio of the controlled flow of the source gas to the flow ofun-activated NH₃, from 10:1 to 5:1 reduces the selectivity of thepolysilicon etch rate over the silicon dioxide etch rate from 20:1 to2:1.
 4. The tunable plasma etch process of claim 1, wherein decreasingthe ratio of the controlled flow of the source gas to the flow ofun-activated NH₃ reduces a concentration of at least one of F and Hradicals in the etch gas stream.
 5. The tunable plasma etch process ofclaim 1, wherein a ratio of NF₃ to NH₃ in the source gas is greater thanone.
 6. The tunable plasma etch process of claim 1, wherein generatingthe plasma comprises generating a capacitively coupled plasma.
 7. Thetunable plasma etch process of claim 1, wherein the pressure iscontrollable at least across a range of 0.5 Torr to 10 Torr.
 8. Thetunable plasma etch process of claim 7, wherein the ratio of thecontrolled flow of the source gas to the flow of un-activated NH₃ is5:1, the pressure is controlled to 6 Torr and the polysilicon andsilicon dioxide etch rates are such that an etch selectivity of thetunable plasma etch process is about 3:1, silicon dioxide over poly. 9.The tunable plasma etch process of claim 1, further comprising addingoxygen to the source gas, prior to generating the plasma.
 10. Thetunable plasma etch process of claim 1, further comprising adding one ofhelium and argon to the source gas, prior to generating the plasma. 11.The tunable plasma etch process of claim 1, further comprisingcontrolling a temperature of a workpiece on which the etch rates of theat least one of polysilicon and silicon dioxide are adjusted.
 12. Thetunable plasma etch process of claim 11, further comprising controllingthe temperature of the workpiece within a range of 100 C to 125 C.